The present invention relates to a lithography technique, more specifically, to a plasma etching method for use in the field of manufacturing semiconductor devices. The invention also relates to a method of manufacturing a photomask with a phase-shift pattern formed by the plasma etching method and to a plasma etching apparatus for use in manufacturing the photomask.
Recently, further development of miniaturization techniques for processing highly integrated semiconductor devices has been strongly desired. Under these circumstances, many trials have been made to improve a photomask and an optical system of a stepper, with the intention of increasing pattern resolution and depth of focus for the exposure light.
As one of these trials concerning the photomask, a phase shifting mask has been developed which has phase shifters selectively arranged on the surface of a mask pattern. An optical image partially inverted by the phase shifter and arranged next to a non-inverting portion. Therefore, the resolution and the depth of focus of an optical image can be improved. As such a phase shifting mask, an alternated phase shifting mask, half-tone mask, shifter-edge-mask, and self-aligning mask have been reported. Of these phase shifting masks, the alternated phase shifting mask (disclosed by M. D. Levenson et al.) is the most effectively improved in resolving power and depth of focus.
As known well, the alternated phase shifting mask, comprises a transparent substrate and an opaque film provided on the transparent substrate. The opaque film has a plurality of openings over which the phase shifters are alternately provided. Light transmitted through the phase shifter is phase-inverted relative to the light transmitted through the opening with no phase shifter. The phase shifter used herein is formed by providing a shifter material such as SOG (spin-on-glass) to a mask having an opaque film pattern provided thereon (made of Cr etc.) to a thickness d, and patterning the resultant film. The thickness d is defined by the wavelength of exposure light as shown in the following equation: EQU d=.lambda./2(n-1)
where n is the refractive index of a shifter material and .lambda. is the wavelength of exposure light.
The structure of the phase shifter thus obtained is known as an additive shifter type.
However, the phase shifting mask of the additive shifter type has the following problems. Since an additional step for forming the shifter material is required, the manufacturing process is complicated. In addition, a phase shifter is easily removed during a washing step of the mask depending the properties for the shifter material. If the phase shifter and the mask substrate differ in refractive index, exposure light is reflected at the interface between them, degrading an optical image including resolution and a depth of focus.
There is another type of the alternated phase shifting mask called "subtractive shifter type" (Jpn. Pat. Appln. KOKAI Publication No. 62-189468). In this type of the photomask, the phase shifter is formed by etching a mask substrate made of quartz to the thickness d defined above. To be more specific, a mask is first prepared by providing an opaque film pattern (Cr etc.) on a transparent substrate (quartz etc.). Then, a resist pattern is provided over alternate openings (light-transmitting portion) of the opaque film. Thereafter, the substrate is etched by RIE (Reactive Ion Etching) etc. using the resist pattern as a mask, thereby preparing a phase shifter. After the resist is removed, the alternated phase shifting mask of a subtractive shifter type is obtained.
In the shifter-edge type mask, openings are formed on a transparent substrate by using a resist pattern instead of the opaque film. After the substrate exposed in the openings is etched by RIE etc., the resist is removed, thereby forming the shifter-edge type mask.
Furthermore, a phase shifting mask of a dual trench shifter type has been developed as another type of the subtractive shifter mask. The dual trench shifter mask is formed by etching alternate openings followed by etching the entire surface of a light transmitting portion (quartz substrate) of the mask uniformly.
As described above, in the conventional phase shifting mask of the subtractive shifter type, to form a phase shifter, a resist pattern of the shifter portion is written and developed in a single step, and thereafter the engraving (subtracting) portion is uniformly etched by use of an etching apparatus, or thereafter the resist is removed and the entire surface of the mask is etched.
However, it is very difficult to obtain the same etching depth in all shifter patterns since the etching rate varies depending on the etching sites by the reasons intrinsic to the etching apparatus and varies depending on the dimensions of the opening of the pattern to be etched (the microloading effect).
To describe more specifically, in the RIE etching apparatus using a plasma generated between parallel plate electrodes, there is a problem caused by the factors intrinsic to the apparatus, such as the electrode structure, plasma generation method, and processing conditions. That is, an etching rate of the center potion of a work piece (which is a member to be etched) closer to the electrode is higher or lower than the peripheral portion thereof. There is another problem with the etching rate as depicted in FIG. 1 which shows the relationship between the pattern size of the work piece and the etching rate (relative value). That is, the etching rate decreases, as the pattern size becomes smaller.
To increase the depth of focus by improving a resolution of an optical image by use of phase-inversion, the shifting amount shifted by the phase shifter must be close to 180.degree.. If the phase is shifted more than or less than the desired value (180.degree.), the intensity of light transmitted through adjacent openings differs at the time of defocusing, as shown in FIGS. 2A and 2B (phase shift: 170.degree.). In FIG. 2B, the position on a mask corresponds to the lateral position of the mask shown in the cross sectional view of FIG. 2A. In FIG. 2A, reference numeral 1 denotes a transparent substrate, reference numerals 2 and 3 indicate a subtractive shifter portion and a non-shifter portion, respectively, and reference numeral 4 represents an opaque film.
In the case where a light exposure apparatus has a focus error or in the case where a stepped portion present in the surface of a substrate, the pattern cannot be formed with an accurate dimensions, deteriorating properties of a semiconductor device. The phase shifting amount is principally proportional to the etching depth d of the substrate. It is therefore necessary to control the etching depth accurately in such a way that the etching is performed at a nearly desired value in the step of manufacturing a light-exposure mask.
To sum up, to control the etching depth highly precisely in the light-exposure mask having the phase shifter, it is necessary to reduce non-uniformity in etching rate which varies depending on etching sites and to reduce variation in etching amount (etching depth) due to the microloading effect.
In the etching apparatus, the difference in etching rate between the center portion and the peripheral portion of the substrate mounted on an electrode leads to the difference in etching depth between the central portion and the peripheral portion of the semiconductor wafer, decreasing the reliability of the semiconductor device.